Parent container

Child container

box-sizing: content-box;
width: 100%;
padding: 0;
border-width: 0;

Parent container

Child container

box-sizing: content-box;
width: 100%;
padding: 5px;
border-width: 10px;

Parent container

Child container

box-sizing: border-box;
width: 100%;
padding: 5px;
border-width: 10px;